1. Field of the Invention
The present invention relates to a transmitter circuit in a communications device such as a mobile telephone or a wireless LAN device and an electronic device such as an audio device or a video device, and to a data converter section and a data conversion method for use therein. More particularly, the present invention relates to a transmitter circuit, a communications device and an electronic device that are capable of suppressing quantization noise, small in size and capable of operating with a high efficiency, and to a data converter and a data conversion method for use therein.
2. Description of the Background Art
A communications device provided on the terminal side of a mobile communications system, such as a mobile telephone or a wireless LAN device, is expected to operate with a low power consumption while ensuring the linearity in the output signal over a wide power amplification range. Accordingly, there is a demand for such a communications device to have a transmitter circuit capable of ensuring the linearity in the output signal while being capable of power amplification of the output signal with a low power consumption.
FIG. 33 is a block diagram showing an exemplary configuration of a conventional communications device. Referring to FIG. 33, the conventional communications device includes a transmitter circuit 900, a receiver circuit 951, an antenna duplexer 952 and an antenna 953. A high-frequency signal to be transmitted is produced at the transmitter circuit 900, and is radiated into the air from the antenna 953 via the antenna duplexer 952. A high-frequency signal received by the antenna 953 is passed to the receiver circuit 951 via the antenna duplexer 952, and the received signal is processed. The antenna duplexer 952 may be a duplexer using, for example, a switch, a dielectric, a SAW (Surface Acoustic Wave) filter, an FBAR (Film Bulk Acoustic Resonator) filter, etc.
An example of the transmitter circuit 900 used in a conventional communications device is a transmitter circuit disclosed in Japanese Laid-Open Patent Publication No. 2002-325109. FIG. 34 is a block diagram showing an exemplary configuration of the conventional transmitter circuit 900. The transmitter circuit 900 shown in FIG. 34 will be hereinafter referred to as a first conventional transmitter circuit. Referring to FIG. 34, the first conventional transmitter circuit includes a data production section 901, an output terminal 902, a delta-sigma modulator 903, an angle modulator section 904, a voltage controller section 905, an amplitude modulator section 906 and a bandpass filter 907.
The data production section 901 produces amplitude data and phase data as data to be transmitted. The amplitude data is inputted to the delta-sigma modulator 903. The delta-sigma modulator 903 delta-sigma-modulates the received amplitude data to output delta-sigma-modulated data. The delta-sigma-modulated data is inputted to the voltage controller section 905. The voltage controller section 905 supplies, to the amplitude modulator section 906, a voltage that is controlled according to the delta-sigma-modulated data.
The phase data is inputted to the angle modulator section 904. The angle modulator section 904 angle-modulates the received phase data to output an angle-modulated wave signal. The amplitude modulator section 906 amplitude-modulates the angle-modulated wave signal with the voltage supplied from the voltage controller section 905 to produce a modulated wave signal. The modulated wave signal produced by the amplitude modulator section 906 is inputted to the bandpass filter 907. The bandpass filter 907 removes out-of-band, unnecessary portions from the modulated wave signal produced by the amplitude modulator section 906. The modulated wave signal whose unnecessary components have been removed through the bandpass filter 907 is outputted via the terminal 902.
In a case where amplitude data is delta-sigma-modulated into two values of zero and a real number, for example, the delta-sigma modulator 903 outputs delta-sigma-modulated data that is discretized into two values of zero and a real number. Thus, the delta-sigma-modulated data, which has only two states (i.e., ON and OFF) with a constant ON-state level, is a signal that is unlikely to be influenced by the non-linearity of the amplitude modulator section 906. Therefore, the first conventional transmitter circuit can output a transmitted signal with little distortion.
Another example of the transmitter circuit 900 used in a conventional communications device is a transmitter circuit disclosed in Japanese Laid-Open Patent Publication No. 2004-072734. FIG. 35 is a block diagram showing an exemplary configuration of the conventional transmitter circuit 900. The transmitter circuit 900 shown in FIG. 35 will be hereinafter referred to as a second conventional transmitter circuit. Referring to FIG. 35, the second conventional transmitter circuit includes an input terminal 911, a data converter section 912, an amplifier 913, a bandpass filter 914 and an output terminal 915.
The data converter section 912 performs a predetermined data conversion operation on an input signal received via the input terminal 911 to output a signal having a smaller resolution magnitude-wise than that of the input signal. Specifically, the data converter section 912 delta-sigma-modulates amplitude data, which is the amplitude component contained in the input signal, and multiplies the delta-sigma-modulated amplitude data with phase data, which is the phase component contained in the input signal, to produce the signal having a lower resolution than that of the input signal. The amplifier 913 amplifies the output signal from the data converter section 912, and outputs the amplified signal to the bandpass filter 914. The bandpass filter 914 removes quantization noise introduced by the delta-sigma-modulation from the signal amplified through the amplifier 913. The signal from which the quantization noise has been removed is outputted via the output terminal 915.
However, the first and second conventional transmitter circuits have a problem as will be described later. The problem will now be described with respect to the first conventional transmitter circuit (see FIG. 34). In the first conventional transmitter circuit, the modulated wave signal outputted from the amplitude modulator section 906 contains quantization noise introduced by the delta-sigma-modulation at frequencies relatively near the intended wave frequency. FIG. 36 shows an exemplary waveform of the modulated wave signal outputted from the amplitude modulator section 906. In FIG. 36, the frequency along the horizontal axis represents the frequency shift with respect to the center frequency (intended wave frequency) of the modulated wave signal. As can be seen from FIG. 36, the modulated wave signal contains a high level of quantization noise at frequencies relatively near the intended wave frequency. Therefore, the bandpass filter 907 for removing quantization noise desirably has steep attenuation characteristics. With the bandpass filter 907 having such characteristics, it is difficult to reduce the signal loss therethrough or to reduce the physical size thereof.
Moreover, when the transmitter circuit changes the transmission frequency band, the bandpass filter 907 having steep attenuation characteristics needs to frequently switch the band of a signal to be passed therethrough (the pass band) from one to another. FIG. 37 shows exemplary characteristics required for the bandpass filter 907 used in the conventional transmitter circuit. Referring to FIG. 37, the bandpass filter 907 switches the pass band from one to another among pass bands A to D each time the transmitter circuit changes the transmission frequency band. In order to realize characteristics as shown in FIG. 37, the bandpass filter 907 needs to be provided with a variable-capacitance device such as a varactor, for example. However, the bandpass filter 907 has a problem that the varactor may cause a loss or an increase in the circuit scale.
In order to reduce the quantization noise introduced by the delta-sigma-modulation in the conventional transmitter circuit without requiring steep attenuation characteristics for the bandpass filter 907, it is then necessary to, for example, increase the clock frequency of the delta-sigma modulator 903. However, the delta-sigma modulator 903 has a problem that an increase in the clock frequency may cause an increase in the power consumption and an increase in the circuit scale. Thus, the first and second conventional transmitter circuits have a problem that it is necessary to use the bandpass filter 907 having steep attenuation characteristics, the delta-sigma modulator 903 operating at a high clock frequency, etc., whereby it is difficult to reduce the circuit scale and the power consumption.
Another example of the conventional transmitter circuit 900 that solves these problems is a transmitter circuit disclosed in Japanese Laid-Open Patent Publication No. 2004-072735. FIG. 38 is a block diagram showing an exemplary configuration of the conventional transmitter circuit 900. The transmitter circuit 900 shown in FIG. 38 will be hereinafter referred to as a third conventional transmitter circuit. Referring to FIG. 38, the third conventional transmitter circuit includes an input terminal 921, a signal processing section 922, a first signal source 923, a second signal source 924, a main amplifier 925, an auxiliary amplifier 926, a synthesizer 927 and an output terminal 928.
Based on a signal X0 inputted via the input terminal 921, the signal processing section 922 produces a signal X1 to be outputted to the first signal source 923 and a signal X2 to be outputted to the second signal source 924. The signal X1 to be outputted to the first signal source 923 is obtained by delta-sigma-modulating the input signal X0. The signal X2 to be outputted to the second signal source 924 is obtained by removing the input signal X0 from the signal X1 to be outputted to the first signal source 923.
The first signal source 923 produces an analog signal taking two or more discrete values, and outputs the produced signal to the main amplifier 925. For example, the output signal from the first signal source 923 is a two-valued analog signal obtained by delta-sigma-modulating the input signal X1 inputted to the first signal source 923, and is a signal containing a component of the input signal X1 inputted to the first signal source 923 and quantization noise components introduced by the delta-sigma-modulation. The second signal source 924 outputs a signal equivalent to the quantization noise components of the output signal from the first signal source 923.
The main amplifier 925 amplifies the signal outputted from the first signal source 923, and outputs the amplified signal to the synthesizer 927. The auxiliary amplifier 926 amplifies the output signal from the second signal source 924, and outputs the amplified signal to the synthesizer 927. Based on the output signals from the main amplifier 925 and the auxiliary amplifier 926, the synthesizer 927 removes the quantization noise from the output signal of the first signal source 923. Specifically, the synthesizer 927 makes an adjustment so that the quantization noise contained in the output signal from the first signal source 923 and the output signal from the second signal source 924 (i.e., quantization noise) are equi-amplitude anti-phase signals, and synthesizes these signals together, thus removing the quantization noise from the output signal of the first signal source 923. The synthesized signal from the synthesizer 927 is outputted via the output terminal 928. Thus, the third conventional transmitter circuit can suppress quantization noise without using a bandpass filter having steep attenuation characteristics, a delta-sigma modulator operating at a high clock frequency, or the like.
However, in the third conventional transmitter circuit (see FIG. 38), the signal delta-sigma-modulated through the first signal source 923 (i.e., a signal containing an intended wave signal and quantization noise) and the signal produced by the second signal source 924 (i.e., quantization noise) are separately amplified and then synthesized together on an analog basis through the synthesizer 927 to remove the quantization noise. Therefore, the third conventional transmitter circuit has long signal paths before the synthesizer 927 where the two signals are synthesized together, and has many analog components. Thus, a troublesome control is required for ensuring a match between the levels, phases and delays of the signals traveling along these paths, and it is difficult to sufficiently reduce the circuit scale and the power consumption.